Valveking 112 heater voltages

[GainFreak]

Hi,

So we have a 112 with a pair of RUBY tubes.
The heater voltage going to the power board is ~19.8VDC (at 232VAC line voltage).
V5 reads 7.4VDC across the heater, V4 - 6.4VDC, V3 - 6VDC. I'm worried about that 1V difference which is out of specs.
Both tubes are drawing ~18mA but the paint on the one with 7.4VDC across heater is more "baked" than the other.
I tried a different 6L6GC tube there and I get almost the same voltage reading.
Any ideas and/or comments would be appreciated.

[Enzo]

Did you try swapping places with the two 6L6s? See if the difference moved with the tube?

If the two tubes are conducting about the same current, then I'd probably not sweat it.

[GainFreak]

Yes, I tried. The same tube has the issue. I also tried another pair and the result was something like 7V2/7V1/5V6(V3).
Tried one more pair (all different brands) - 6V9/6V5/6V2.

[Enzo]

So it is just variations between tubes. There is little you can do about it. The voltage across each one is proportional to the current through it and its internal resistance - Ohm's Law. The tubes are in series, so the current is the same through each.

Ever play on some amp that has had the same set of tubes in it since Elvis was a kid? AMp stillworks, sounds kinda dead, like dead strings, but it works. You put fresh tubes in it, and it sings. ANy life shortening by having your heater voltage 15% high will occur long after you would have replaced them anyway.

[GainFreak]

Maybe you're right however 7.4V is still kind of high and out of range.
For the sake of "purity" how about adding 3 equal value resistors (10k?) across each of the 3 heaters like for the caps in series in a HV supply? This way they won't affect the total current draw but will distribute those 19.8VDC evenly across the tubes' heaters?

[Enzo]

And then when different tubes go in it?

[GainFreak]

Actually it was a wrong idea because those resistors will be in parallel with the heaters so it won't work as I thought initially.
I guess the only way is to select tubes for equal heater voltage in series but I'll leave that to somebody else.

[Enzo]

Honestly, the only reason you even know about this condition is you were probing around measuring stuff. It won't affect operation or reliability.

[g1]

Honestly, the only reason you even know about this condition is you were probing around measuring stuff.

And because of the series heaters. If they were standard parallel arrangement they would each have the same voltage across them. No one would ever think of breaking into the circuit and measuring the current through each.

[Enzo]

Well, that is true. (Imagining 1 ohm resistors in series with each heater, and...)

The only time I measure across a heater is to see if the voltage is present. And only then if the tube is dark.